Microsoft, not surprisingly, has been spending a lot of time mulling over that question in recent years. Its touch-centric new operating system, Windows 8, is largely devoted to answering it. And for the first time, the company decided to show us exactly what it thinks a modern PC-tablet hybrid should be by designing and selling its own Windows computer, Surface.

Except it didn’t come up with one Surface — it built two of them. The first version, Surface Windows RT, shipped in October, simultaneously with Windows 8. Technically speaking, however, it isn’t a Windows 8 machine: it uses a power-efficient ARM processor and a special version of Windows called Windows RT that only runs new programs designed for the touch-friendly “modern” interface, not all the apps written for conventional PCs. Starting at $499, it’s the closest thing Microsoft has to a direct iPad competitor.

And then there’s Surface Windows 8 Pro, which goes on sale at the Microsoft Store, Best Buy, microsoft.com and elsewhere on Feb. 9. (That’s Microsoft’s full official name for it; I hope the company won’t be irked if I refer to it as “Surface Pro,” like everyone else is already doing.) It has much in common with Surface RT: hold one Surface in each hand, and the only hint that they’re not the same device is the Pro version’s additional bulk — it’s 0.53 in. (1.3 cm) thick and weighs 2 lb. (0.9 kg), vs. Surface RT’s 0.37 in. (0.9 cm) and 1.5 lb. (0.7 kg).

Both versions have an elegant vapor-magnesium case and kickstand that props it up for vertical use, and both work with the same whisper-thin Touch Cover, which includes a keyboard that’s nearly flat yet reasonably comfy. Both have 10.6-in. (27 cm) screens, though the Pro’s version, at 1920-by-1080 resolution, packs additional pixels.

But Surface Pro, unlike Surface RT, is a real PC. As its name indicates, it comes with Windows 8 Pro, Microsoft’s top-of-the-line operating system. It sports a powerful Intel Core i5 processor, 4GB of RAM, either 64GB or 128GB of solid-state storage and a Mini DisplayPort port; you could hook it up to a keyboard, mouse and external display, run any program you throw at it, use it with any Windows-compatible hardware add-on and generally forget that you’re sitting in front of anything but a brand-new conventional PC.

Of course, if that were all you intended to do, you’d be better off simply buying a conventional PC. Surface Pro’s potential lies in its ability to segue between multiple computing scenarios: you can use it like a desktop PC, or a laptop or a tablet. Unlike a Windows RT device, it could be a typical consumer’s sole computer.

Microsoft likes to use the phrase “no compromises” when describing that versatility, but in fact, Surface Pro — like all computing devices — is a study in compromises. It’s bulkier than Surface RT because its components require more interior space. Microsoft’s stated battery life is five hours, compared with eight for Surface RT. Even the AC adapter is portlier. (One nice touch: it sports an additional USB port you can use to recharge your phone or another gizmo.)

The price is heftier too. Surface Pro comes in a 64GB version for $899 or a 128GB one for $999; virtually everyone who buys one will also want either a $120 Touch Cover or a $129 Type Cover. (The latter isn’t as thin as the Touch Cover, but it has real clacky keys that move when you press them.) That puts them in the same price range as the cheaper versions of Apple’s MacBook Air and some of the slicker Windows 8–based Ultrabooks.

In fact, Surface Pro qualifies, specs-wise, as an Ultrabook, even though its substitution of a kickstand and removable keyboard cover for a traditional clamshell case is unique. It works well for the most part, although if you balance the whole setup on your lap, you may find it slightly precarious. If you think of this as an ultraportable PC rather than a tablet, it suddenly gets sexier — it’s thin, light, and well engineered. (It has two interior fans for cooling, but I’ve never heard them at work.)

Those 64GB and 128GB capacities are a tad misleading, though: Microsoft preinstalls a full copy of Windows 8 and reserves space for a recovery partition, gobbling up a major percentage of your space. The 64GB Surface Pro offers 23GB of usable space, which will be tight if you plan on installing many of the third-party programs that give this product its purpose. The 128GB model has a relatively bountiful 83GB. Both models have a microSD slot that allows you to expand the memory; a 64GB card will run you around $60.

[UPDATE: ZDNet’s Ed Bott reports that the 128GB Surface Pro actually has 89.7GB of free space, and that the numbers I quoted were based on a pre-release version of Surface Pro.]

Unlike Surface RT, the Pro models don’t come with a bundled copy of Office 2013. But they do include a digital pen based on Wacom’s pressure-sensitive technology. You can use it for sketching, note-taking and handwriting or simply as a mouse substitute; it doesn’t require batteries, but cleverly attaches to the tablet’s magnetic power connector when not in use.

Hit Surface Pro’s power button, and you get a little reminder right away that it’s more PC than tablet: Unlike Surface RT, which springs to life more or less instantly, it takes a few seconds to wake up. But then you land in Windows 8’s start screen, which looks exactly like Windows RT’s version. Bundled apps like Internet Explorer work the same; so does the Windows Store, which is gradually filling up with third-party apps.

If you’ve chosen a Surface Pro over Surface RT, it’s because you want to run conventional Windows apps. You do that in the Desktop, the section of Windows 8 that is essentially a modestly refreshed version of Windows 7 stripped of its iconic Start button. I installed a selection of them, including the Office 2013 suite, Adobe’s Photoshop CS6, Chrome, Firefox, iTunes, TweetDeck and more.

And over and over again, these useful programs reminded me that they weren’t designed to work well on a new-wave computing device like the Surface Pro.

When I first tried Chrome, it acted like it had taken leave of its senses: when I tapped on one message in Gmail’s inbox, a different message would open up. According to Microsoft, that’s because the current version of Chrome wasn’t written to use touch input properly on a high-dots-per-inch screen like the one on the Surface Pro. Neither of the workarounds Microsoft suggested — running Chrome in full-screen mode with teensy type or avoiding using the touchscreen — made the browser truly usable. (Google can presumably fix this with an update, and I hope it will.)

Surface Pro has enough raw horsepower to run Photoshop well, but the application’s menus and icons are minuscule on the 10.6-in. (27 cm) display; tapping the right feature felt like threading a needle, even when I used the pen rather than my fingertip. Firefox looked good at first blush, but getting taps to register often required multiple jabs, and some websites, such as nytimes.com, loaded with microscopic type and too much white space.

Even Microsoft’s own Office 2013 programs — the only ones with interfaces that were in fact rejiggered slightly to be more finger-friendly — aren’t really pleasant to use with the touchscreen. I ended up using them mostly with the Touch Cover’s touch pad. It’s not bad given its matchbook-like dimensions, but Windows 8 works best with a much roomier touchpad.

Now, using a mouse with Surface Pro would eliminate most of the issues I encountered — and Microsoft supplied the review unit I tested with its Wedge Touch Mouse Surface Edition, a product that acknowledges at least some Surface owners won’t be ready to go mouse-free. It’s also possible that updates from third-party developers will make their wares more usable on Surface. They’re going to need to think more about touch: Intel plans to make touchscreens a mandatory feature on the next generation of Ultrabooks.

I’m not arguing that Surface with Windows 8 Pro is a machine without a market. If you equip it with an external display, keyboard and mouse, it becomes a serviceable desktop PC, and if you stick to Windows 8 apps, it may be the best Windows 8 tablet so far. If I were shopping for an Ultrabook and my budget allowed, I’d consider it. But used with the applications I tried, Surface Pro doesn’t prove that one computing device can do everything well. Instead, it makes clear that there’s no such thing as no-compromise computing.

That’s not the lesson Microsoft intended, but it’s a useful one nonetheless — for consumers, for the industry and maybe even for Microsoft.